1. Field of the Invention
The present invention relates to a method for communicating across an electrical isolation barrier, and in particular, to a method and apparatus utilizing Peltier heating and the Seebeck Effect to permit communication of a thermal signal across an electrical isolation barrier.
2. Description of the Related Art
Electrical isolation boundaries are generally employed either to eliminate interference between electrical circuits, or to provide a current barrier for safety purposes. One example of an electrical isolation barrier is found in a power supply which runs off of a wall plug, but which has outputs electrically isolated from the wall plug itself. Another example of the utilization of an electrical isolation barrier is found in the interface to a telephone system. Voice, data, and supervisory functions requiring a power supply to operate must be able to transmit signals between telephone lines and telephone or computing equipment while maintaining at least 1000 VRMS electrical isolation between the user and the power supply. This isolation ensures the safety of the user pursuant to FCC regulations.
A variety of methods have previously been employed to permit communication between systems separated by an electrical isolation barrier. All of these methods involve conversion of electricity to some other form of energy, which is transmitted across the boundary and then reconstituted in the form of an electrical signal.
Optoisolators convert an electrical signal into radiant energy, and then transmit the radiant energy across the electrical isolation barrier. Optoisolators consist of a light emitter and a light detector sealed together in a light-tight package. The intensity of the light emitted varies according to the amplitude of the electrical signal applied to the light emitter. The light detector receives the emitted light and in turn produces an electrical output having an amplitude corresponding to the intensity of the light received. The emitter and detector are manufactured as two separate elements, and are separated by an electrical isolation barrier in the form of a transparent or translucent electrically insulating material such as glass or plastic.
Another manner of communicating across an electrical isolation barrier is by the use of a transformer. Transformers convert an electrical signal into a magnetic field using a first electrically conducting coil. A second electrically conducting coil is then exposed to the magnetic field, either by virtue of physical proximity to the first coil or by directing the magnetic field using a magnetic material or xe2x80x9ccore.xe2x80x9d The second coil produces an electrical output in response to changing intensity of the magnetic field.
While optoisolators and transformers function adequately in many applications, these structures present a number of disadvantages. Most importantly, optoisolators and transformers cannot readily be integrated into an integrated circuit without utilizing expensive processing techniques. This is because complex and specialized structures are required to perform light emission and detection functions, or to generate and detect magnetic fields. As a result, optoisolators and transformers are generally only available in discrete form, and thus present economic and volumetric penalties to system designers.
Therefore, there is a need in the art for a device which enables systems to effectively communicate across an electrical isolation barrier but which avoids the complexity and cost of existing coupler technologies.
U.S. Pat. No. 4,757,528 to Falater et al. (xe2x80x9cthe Falater Patentxe2x80x9d) describes a coupler that utilizes transmission of a thermal signal in the form of Joule""s heat across an electrical isolation barrier. Specifically, the Falater Patent describes the application of current to a resistor positioned on one side of the boundary. Passage of current through the resistor generates a thermal signal in the form of a temperature increase in the resistor and the surrounding material. This temperature increase may propagate across the electrical isolation barrier and be detected by changes in base-emitter voltage of a bipolar transistor positioned on the other side of the boundary.
The thermal coupler disclosed by the Falater Patent offers the advantage of utilizing components such as resistors and bipolar transistors that are readily incorporated in an integrated circuit. However, the Falater Patent suffers from a major disadvantage in that a thermal signal may be transmitted across the boundary only in the form of a temperature increase. Thus, the bandwidth of communication across the electrical isolation barrier is necessarily constrained by upper temperature limits inherent in the system. These temperature limits may be dictated by such factors as the composition of the resistor, the composition of the electrical isolation barrier, the intended use of the device, and the heat tolerance of other semiconducting structures present in the device.
Moreover, the effective bandwidth available to the device described in the Falater Patent would be expected to shrink over time. This is because repeated transmission of signals across the boundary would cause the resistor and the surrounding material to heat up, thereby narrowing the range of temperature changes detectable over an increasingly hot background.
Therefore, there is a need in the art for a device that enables a thermal signal to be effectively communicated across an electrical isolation barrier without the limitations in bandwidth present in existing designs.
The present invention relates to a thermal coupling device that utilizes Peltier heating and cooling to transmit a thermal signal across an electrical isolation barrier. The thermal coupler includes a thermal emitter and a thermal detector, both in the form of a first electrically conducting material separated by a second electrically conducting material. Application of a voltage from a power supply across the first electrically conducting material of the thermal emitter results in Peltier heating at junctions between the first and second electrically conducting materials. This temperature difference is transmitted across the electrical isolation barrier to a thermal detector positioned on the other side. The thermal detector has a structure similar to that of the thermal emitter. However, the thermal detector does not bear an applied voltage. Differential heating of junctions between the first and second conducting materials of the thermal detector, due to their proximity to the junctions of the thermal emitter, give rise to a Seebeck voltage. This Seebeck voltage can be amplified and detected. In this manner, Peltier heating and the Seebeck Effect are utilized to communicate a thermal signal across an electrical isolation barrier.
A thermal-coupler in accordance with one embodiment of the present invention comprises a thermal emitter positioned on one side of the electrical isolation barrier, the thermal emitter including a first electrically conducting material coupled to a second electrically conducting material. The thermal emitter experiences a voltage difference applied from a power supply, and generates a thermal signal by Peltier heating. This thermal signal propagates across the electrical isolation barrier to an opposite side. The thermal coupler further comprises a thermal detector positioned on the opposite side of the electrical isolation barrier and includes a third electrically conducting material coupled to a fourth electrically conducting material. The thermal detector receives the thermal signal and in response generates a Seebeck voltage signal.
A method of communicating a thermal signal across an electrical isolation barrier in accordance with the present invention comprises the steps of providing a first junction between a first set of dissimilar electrically conducting materials positioned on one side of the electrical isolation barrier. A second junction is provided between the first set of dissimilar electrically conducting materials. A third junction is provided between a second set of dissimilar electrically conducting materials positioned on an opposite side of the electrical isolation barrier, the third junction proximate to the first junction. A fourth junction is provided between the second set of dissimilar electrically conducting materials, the fourth junction proximate to the second junction. A potential difference is applied across the first set of dissimilar electrically conducting materials to cause Peltier heating of the first junction and Peltier cooling of the second junction. A Seebeck voltage generated between the third junction and the fourth junction due to a temperature difference attributable to Peltier heating of the first junction and Peltier cooling of the second junction is then detected.
The features and advantages of the present invention will be understood upon consideration of the following detailed description of the invention and the accompanying drawings.